Merchant was the first wife of Harold Pinter, whom she met while working as a repertory actress; he was then working as an actor under the stage name of David Baron. They married in 1956, and their son, Daniel, was born in 1958.
Their marriage began disintegrating in the mid-1960s. From 1962 to 1969, Pinter had a clandestine affair with Joan Bakewell, which inspired his play Betrayal. In 1975, he began a serious affair with the historian Lady Antonia Fraser, the wife of Sir Hugh Fraser, which he confessed to his wife that March. At first, Merchant took it very well, saying positive things about Fraser, according to her friend artist Guy Vaesen (as cited by Billington); but, Vaesen recalled, after "a female friend of Vivien's trotted round to her house and poisoned her mind against Antonia ... life in Hanover Terrace [where the Pinters then lived] gradually became impossible". Pinter left, and Merchant filed for divorce and gave interviews to the tabloid press, expressing her distress. Merchant made some unflattering comments about Fraser at this time: "He didn't need to take a change of shoes. He can always wear hers. She has very big feet, you know." Merchant believed Fraser to be the basis for the character of Emma in Pinter's play Betrayal, never learning about his prior affair with Joan Bakewell.